abby ren, princeton daniel sigman, princeton nele meckler, caltech rebecca robinson, uri

Abby Ren, Princeton

Daniel Sigman, Princeton

Nele Meckler, Caltech

Rebecca Robinson, URI

Yair Rosenthal, Rutgers

Gerald Haug, ETH

Foraminifera-bound nitrogen isotopes evidence for reduced nitrogen fixation in the Atlantic Ocean during the last ice age

Pleistocene glacial/interglacial cycles

(C,N,P)org NO

3

-,PO4

-3=0

atmosphere

CO2, NO3

-, PO4

-3

Low-Latitude Biological Pump

thermocline

Biological pump

Low Latitude: Change nutrient inventory

High Latitude: Change nutrient

consumption

Low vs. High Latitude

(C,N,P)org NO

3

-,PO4

-3=0

atmosphere

CO2, NO3

-, PO4

-3

Low-Latitude Biological Pump

thermocline

Low-latitude biological pump

~3 kyr ~15-80 kyr

N2

N2

denitrification

N fixation

Broecker, 1982

N isotopes in nature: 14N = 99.64% 15N = 0.36%

δ

15

(‰ . ) N vs air =

(

15

/N

14

)N

sample

(

15

/N

14

)N

air

- 1

⎛

⎝

⎜

⎜

⎞

⎠

⎟

⎟

* 1000

N isotope terms

MS 172 Figure 5

0

5

10

15

20

0 0.5 1 1.5 2

[NO3-] (factor of initial value)

newly fixed Nadded

δ15N~ -2-0‰

water columndenitrification

ε ∼ 20-30‰

sedimentarydenitrification

ε ∼ 0‰

nitrateuptake

ε ∼ 5‰

NO3- δ15N

(‰ . )vs air

Effects of major N fluxes on nitrate δ15N

Lower water column denitrification during LGM

302520151050age (ka)

14

12

10

8

6

4

δ15

(‰ N

. vs

)air

Denitrification increase

Santa BarbaraBasin

Arabian Sea

Chile Margin

Chile: de Pol-Holz et al, 2006SBB: Emmer and Thunell, 2000AS: Altabet et al., 2002

Low δ15N of nitrate in the North Atlantic thermocline: Paleoceanographic utility

[NO3-] (µM)

δ15N of nitrate (‰ vs. air)

Knapp et al., 2005

Planktonic foraminifera

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Orbulina universa

photo: H. Spero Montoya et al., 2002

Study sites

10

8

6

4

2

0

Foraminifera-bound

δ15

(‰ N

. vs

)air

-BarbudaAntiqua

LittleBahama

Bank

GreatBahama

Bank

Indonesia South Pacific South Pacific

Atlantic Pacific

Sargasso Sea subsurface

Indonesia Seasubsurface

Subantarctic

37 15.64 º ' S176 40.02 º ' E

36 22.63 º ' S177 .26.75 º ' E

6 46.25 º ' S116 58.47 º ' E

Mode Water

. G ruber . G sacculifer . O universa . G menardii . N dutertrei . G truncatulinoides . H pelagica . G inflata

Thermocline nitrate vs. coretop foraminifera

Ren et al., 2009

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

302520151050 ( )age ka

-2

-1

0

δ18

O C

(‰

. vs

)VPDB

6

5

4

3δ15

(‰ N

. vs

)air

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

a.

b.

c.

d.

individual species

bulk sediment

mixed species

O. universa G. sacculifer G. ruber

> 355 µm 250~355 µm 125~250 µm

δ18O, age:Schmidt et al., 2004

LGMHolocene

ODP site 999A:

Caribbean Sea

Ren et al., 2009

1000

800

600

400

200

depth (m)

6543210-1δ15 (‰ N . vs )air

100

80

60

40

20

0

. G ruber

. G sacculifer

. O universa

3.1‰

Modernδ15 :N suspended POM nitrate

- Foraminifera boundδ15 :N Holocene LGM :LGM interpretation

POM: Altabet, 1988NO3

- : Knapp et al., 2005

N fixation rate:LGM/Holocene ~ 20%

Ren et al., 2009

Chile: de Pol-Holz et al, 2006SBB: Emmer and Thunell, 2000AS: Altabet et al., 2002Cariaco: Haug et al., 1998

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

302520151050 ( )age ka

14

13

12

11

10

9

8

7

6

5

4

δ15

(‰ N

. vs

)air

Denitrification increase

N fixation increase

Santa BarbaraBasin

Arabian Sea

Cariaco Basin

:Caribbean Sea . O universa . G sacculifer . G ruber

Chile MarginDeglacial increases

in both denitrification

and N fixation

A stable N inventory regulated by N/P ratio?

Gruber and Sarmiento, 1997

~1P:16N

Fix!

Don’t Fix!

Deglacial scenario:N fixation feedback through N/P sensitivity

Summary

• In Caribbean Sea sediments, planktonic foraminiferal 15N/14N decreases from the last ice age to the current interglacial.

• The foraminiferal change is best explained by less N fixation in the Atlantic during the last ice age, leading to higher nitrate 15N/14N in the Caribbean thermocline.

• The reconstructed increase in N fixation at the end of the last ice age is most likely a response to the previously recognized deglacial increase in global denitrification.

• As with our findings regarding the modern Atlantic N fixation rate, this is consistent with a strong P control on N fixation.

• A significant glacial/interglacial change in the nitrate reservoir has not been ruled out. However, our evidence that Atlantic N fixation acts as a negative feedback argues qualitatively for limits to such a change.

Acknowledgement

• Princeton University • Consortium of Ocean Leadership

• Sigman Group: Franky Wang, Brigitte Brunelle,

Julie Granger, Peter Difore.

• J. Bernhard and D. McCorkle

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Nitrogen cycle

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

302520151050 ( )age ka

-2

-1

0

δ18

O C

(‰

. vs

)VPDB

6

5

4

3δ15

(‰ N

. vs

)air

8

7

6

5

4

3

2

δ15

(‰ N

. vs

)air

a.

b.

c.

d.

individual species

bulk sediment

mixed species

O. universa G. sacculifer G. ruber

> 355 µm 250~355 µm 125~250 µm

Minimal Holocene δ15N

decreasein

bulk sediment

δ18O, age:Schmidt et al., 2004

Ren et al., 2009

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Foraminifera-bound N isotope analysis

•Sieve, pick, crush

•Clean fragments with wet oxidation

•Acid dissolution to release

internal Norg

•Norg NO3- (persulfate oxidation)

•NO3- N2O (denitrifier method)

•N2O isotopic analysis (continuous

flow, purge/trap, gas

chromatography, gas-source magnetic

sector mass spectrometry)

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Carbon inventory

Fig. S1

20

15

10

5

0

N content (µmol/g)

20151050

cleaning time (hours)

7

6

5

4

3

2

1

0

δ15

(‰ N

. vs

)air

( / )N content µmol g δ15 (‰ )N

Fig. S2

7

6

5

4

3

2

1

0

foraminifera-bound N content (µmol/g)

302520151050age (ka)

70

60

50

40

30

20

10

0

bulk sedimentary N content (µmol/g)

O. universa G. sacculifer G. ruber > 355µm 250~355µm 125~250µm bulk N content

Thermocline nitrate/surface sediment comparison

Altabet, 2005

-24.5

-24.0

-23.5

-23.0

-22.5

-22.0

-21.5

δ13

(‰ C

. vs

)VPDB

302520151050 ( )age ka

10

9

8

7

6

5

/ ( )C N weight ratio

/C N δ13 (‰ )C

Downcore changes in δ13Corg and Corg/TN

40

30

20

10

0

TN (µmol/g)

4003002001000Corg (µmol/g)

LGM Holocene

TN=0.0563(±0.0143)*Corg + 21.8695(±4.8225)

R2=0.36

TN=0.0549(±0.0131)*Corg + 14.6769(±4.3894)

R2=0.83

C/N~15 (by weight) terrestrial organics

Evidence for terrestrial and shelf materialin ODP 999A sediments

-22.0

-21.5

-21.0

-20.5

-20.0

-19.5

-19.0

δ13

(‰ C

. vs

)VPDB

30252015105 ( )Age ka

10.0

9.5

9.0

8.5

8.0

7.5

/ ( / )C N g g

45

40

35

30

25

(%)Carbonate content

δ13 (‰ C . vs )VPDB / ( / )C N g g (%)Carbonate content

Fig. S4

Bulk sediment δ15N records from the North Pacific

Kao et al., 2008

Bulk sediment recordsfrom diverse settings:Mean ocean nitrate δ15N did not decrease (much) into the Holocene.

5.5

5.0

4.5

4.0

3.5

3.0

2.5

2.0δ15

. (‰ N of N Atlantic thermocline nitrate

. vs

)air

1.41.21.00.80.60.40.20.0 fraction of modern N fixation

Holocene

LGM

Estimating N fixation change

Surfacechlorophyll

Surfacenitrate

Incomplete nutrient consumption in the polar ocean

WINDS

Siegenthaler (1983)

THERMOCLINE

35NO3-

232CO2

10Norg66Corg

25NO3-

166CO2

›

Eq. S

Rapid surface-deep exchange in the polar ocean releases deeply sequestered CO2 to the atmosphere

Mechanisms for reducing the Southern Ocean CO2 leak

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4000

3500

3000

2500

2000

1500

1000

500

04 4.5 5 5.5 6 6.5 7

4000

3500

3000

2500

2000

1500

1000

500

020 25 30 35 40

[NO3-] (µmol/kg) δ15NO

3- (‰ vs. air)

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0

5

10

15

20

25

30

4

6

8

10

12

14

16

40 45 50 55 60 65 70 75latitude along ~140°E (°S)

0

5

10

15

20

25

30

4

6

8

10

12

14

16

40 45 50 55 60 65 70 75

[NO3-]

δ15NO3-

RITS '94 - central Pacific

The link between theN isotopes and nitrateconsumption

4.0

3.6

3.2

2.8

δ18 O

. ( ) N pachyderma sin

(‰ . )v PDB

100806040200 ( )age ka

6

5

4

3

2- diatom bound

δ15

(‰ . )N v air

δ18O + age: Ninneman and Charles, 1997

• More complete nitrate consumption in glacial Subantarctic

Subantarctic (E11-2) diatom-bound 15N/14N: Link to glacial/interglacial cycles

Mahowald et al., 2005

modern annual average dust deposition (g m-2 yr-1)

Subantarctic is well situated fordust-driven iron fertilization during the

ice ages

LGM

Antarctic 15N/14N:higher during the last glacial maximum

Subantarctic nutrient drawdown:CO2 ~40 ppm

AA deep water formation “off”:CO2 ~35 ppm

Polar AA nutrient drawdown:More CO2 without complete deepwater formation shutdown

Glacial/interglacial changes in the Southern Ocean inferred from the N isotopes

Low and High latitude connection

Lower rates of N fixation